Who determines gender? Men's health knowledge: Chromosome demystification.

Procreation is the primary need for human reproduction and evolution, yet "human understanding of sexual physiology is still less clear than our understanding of the moon," making sex and reproduction one of the biggest misconceptions in human history, rife with superstition and myths.

From a modern scientific perspective, human sex has two layers of attributes.

I. Biological Sex

Sex is determined by the unique reproductive organs and development of males and females; this sex is hereditary.

Throughout history, many hypotheses have been proposed regarding the mystery of male and female offspring, but none have been scientifically proven. The mystery of sex persisted until the 20th century, when chromosomes were discovered, and especially the sex chromosomes were identified, finally revealing the answer.

All living organisms are composed of cells. The cell nucleus contains rod-shaped structures called chromosomes. The number of chromosomes varies among animal species: humans have 46, monkeys 48, dogs 78, rabbits 44, and frogs 8. Chromosomes contain hereditary factors, or "genes." All genetic information resides in genes. Genes are arranged on chromosomes, which are composed of two helical strands of deoxyribonucleic acid (DNA) of different lengths. The smallest unit of DNA is the nucleotide—a type of base. DNA has a very simple structure—only four bases (A-adenine, G-guanine, C-cytosine, and T-thymine). However, because the molecule is very long, the combinations of A, G, C, and T can be infinitely long. Therefore, using the substances represented by these four letters, DNA can "record" a large amount of information, namely various genetic characteristics. The complex structure of DNA forms chromosomes in the cells of various organisms. These chromosomes are also the material carriers of biological heredity, and each locus on a chromosome is "responsible" for a certain genetic characteristic.

Human chromosomes consist of 23 pairs (46 chromosomes), one pair of which are sex chromosomes. Sex chromosomes carry the genetic "code" that determines a person's sex. Females have two identical X chromosomes (XX type), while males have one X chromosome and one smaller Y chromosome (XY type). During development, both sperm and egg cells undergo a unique meiotic division, halving the number of chromosomes from 23 pairs to 23 individual chromosomes.

At fertilization, if an X-type sperm fertilizes the egg, an XX zygote is formed, resulting in a female fetus; if a Y-type sperm fertilizes the egg, an XY zygote is formed, resulting in a male fetus. Therefore, the sex of the fetus is determined by the sperm, not the egg; that is, by the male, not the female.

Incidentally, why do some women always have boys while others always have girls? The key factor is still the sperm. Generally, sperm are alkali-loving and acid-averse. Y-type sperm are far less tolerant of acid than X-type sperm. If the vagina is acidic, the motility of Y-type sperm is limited, and they are only active in an alkaline environment. X-type sperm, on the other hand, are more tolerant of acid and are more active in acidic environments. When a woman suffers from conditions such as cervical erosion, the cervical mucus becomes excessively thick, restricting sperm movement, especially since Y-type sperm are small and unable to penetrate the thick cervical mucus. X-type sperm, however, are more vigorous and outpace Y-type sperm, fertilizing the egg to form a female fetus. It can be seen that human sex is determined by the sperm at the moment of fertilization, while the egg is neutral and passive. Overall, the fertilization of the egg by either type of sperm is random, with an equal chance of forming an XX zygote or an XY zygote.

After fertilization, a new life begins to develop. This new life is not a copy of the father or mother, but a new life with the genetic characteristics and variations of both parents.